The use of dampers to regulate air flow to or from the combustion chamber of a heating unit is known in the art. However, such dampers are typically manually operated, or are part of needlessly complex systems which require separate temperature or pressure sensors or require multiple dampers, or otherwise do not instantly cause each and every change in the amount of negative flue gas pressure to result in an inverse reaction from the damper to regulate the intake of air. The prior art dampers do not establish or exploit a dynamic relationship between negative gas pressure in the flue and a resistive element in the damper in order to automatically and instantly control the amount of intake air entering the combustion chamber.
U.S. Pat. No. 1,449,133 issued Mar. 20, 1923 ("Street") discloses a single damper which controls air flow to both the flue and the combustion chamber. However, the damper is manually operated.
U.S. Pat. No. 5,413,088 issued May 9, 1995 ("Oviatt") discloses a wood burning heat unit. Oviatt discloses drawing outside air through a pipe into a T-connection with branch pipes connected to the chimney and the combustion chamber respectively. The purpose of the pipe connection to the chimney is to divert to the chimney excess intake air that would otherwise enter the combustion chamber, so as to prevent overheating. Multiple dampers in the form of a pair of air pressure actuated flapper valves (78, 80) regulate air flow to each branch pipe respectively. The damper (80) into the combustion chamber is adjusted so that even a small intake air flow will hold it wide open. The damper (78) to the chimney flue is adjusted to a predetermined heavier setting such that a stronger chimney draft is required to open it. Oviatt does not disclose a single damper reacting to all changes in negative flue gas pressure, and to automatically and dynamically adjust intake air accordingly. Rather, damper (80) to the combustion chamber is wide open most of the time and is substantially unaffected by changes in the negative flue gas pressure, while damper (78) to the chimney flue reacts only to dramatic changes in the negative gas pressure of outflow exhaust so as to divert only large amounts of excess intake air in those instances. This multiple damper system permits only clumsy regulation of the volume of intake air. Further, neither damper (78) nor (80) can be manually adjusted to vary their operating range from time to time.
U.S. Pat. No. 1,221,008 issued Mar. 27, 1917 ("Thompson") discloses an invention for an automatic draft control in respect of a boiler. Thompson discloses a system which uses one damper to regulate the intake of air into a combustion chamber and another damper to regulate the chimney exhaust from the combustion chamber, each damper mechanically linked to an apparatus whose position is dependent on the pressure or temperature in the boiler such that the amount of air entering or leaving the combustion chamber is automatically adjusted in accordance with changes in the pressure or temperature in the boiler. However, Thompson does not disclose a means to establish or exploit any form of dynamic relationship between negative gas pressure in the flue and the intake of air into the combustion chamber, let alone specifically an inverse relationship between the two. Rather, Thompson discloses a system where the flue pressure and the air intake are both increased or are both decreased in accordance with a complex mechanism whose position is dependent on the temperature or pressure in a boiler.
U.S. Pat. No. 1,194,011 issued Aug. 8, 1916 ("Greey et al.") also discloses a damper system with separate dampers on the chimney linked to the intake damper by a chain or a cable run through a system of guide pulleys. Greey et al. disclose using a "thermostatic or other motor" to control the dampers using the chain/cable, and in no way disclose using negative flue pressure to automatically operate the air intake damper.
U.S. Pat. No. 4,406,396 issued Sep. 27, 1983 ("Habegger") and U.S. Pat. No. 2,151,512 issued Mar. 21, 1939 ("Hagen") both disclose temperature-regulated combustion air/gas flow methods and apparatus. Habegger discloses using a temperature sensor positioned at the vent between the combustion chamber and the flue in order to control a damper further downstream in the flue so as to regulate the flow of gases through the flue. Habegger does not relates to regulating the intake of air into the combustion chamber.
Hagen is directed toward furnaces used for heating a liquid. Hagen discloses using a thermostatic device to control the intake air damper according to the liquid temperature. Separate thermostatically controlled dampers regulate the chimney outflow.
U.S. Pat. No. 70,979 issued Nov. 19, 1867 ("Eaton") discloses an apparatus which uses the expansion of the stove (B) and the chimney (D) to operate a linkage which controls the air flow through a register (C) and an intake tube (F). When the fire in the stove is being started all the air enters through the register. As the stove and the chimney heat up, they expand and the rod (L) operates on linkage to close the register (C) and open the damper (G) so that intake air is drawn from the upper part of the room, thereby helping to circulate the air in the room.
U.S. Pat. No. 4,180,051 issued Dec. 25, 1979 ("Maier et al.") discloses using a bimetallic thermostat (45) mounted at the front of a furnace to sense the radiant energy from the combustion chamber and to adjust an intake air damper (42) accordingly. However, Maier et al. do not disclose a method or apparatus by which intake air is instantly adjusted in response to changes in negative flue gas pressure.